The phrase "mutual acquaintance" refers to a person that two or more people know or are familiar with. It means that both individuals have a connection or a relationship with the same person. Full definition
I was nineteen when I met my friend Travis through mutual acquaintances in the School of Design at Arizona State University. (faring-well.com)
He sat with us and, as his servants served the tea, gossiped with my friends about mutual acquaintances in Colombo. (offassignment.com)
A new U. S. Congress does not get much done in the first two weeks, even with the advantages of a continuing organization, a body of guiding precedent, a fairly general mutual acquaintance and a single language. (religion-online.org)